A torque converter installed in a transmission of a vehicle is provided with a lockup clutch as a friction element. By setting the lockup clutch in an engaged condition, a deviation between input/output rotation speeds of the torque converter is suppressed, leading to an improvement in the fuel efficiency of an engine.
JP8-28678A discloses a lockup control device for a fluid coupling that suppresses and prevents hunting by setting a determination value for switching the fluid coupling from a converter condition to a lockup condition and a determination value for switching the fluid coupling from the lockup condition to the converter condition on either side of a hysteresis region has been disclosed as this type of torque converter.